Eight monoclonal antibodies (MAbs) to lipoligosaccharides (LOS) of Neisseria meningitidis were produced by immunizing mice with purified LOS from group A meningococcal strain A1. The specificities of the Mabs were examined by enzyme-linked immunosorbent assay (ELIZA), immunodot assay, and ELIZA inhibition using the homologous A1 LOS, 12 immunotype LOS of N. meningitidis (L1 through L12), and LOS or lipopolysaccharides from other gram-negative bacteria. Two of the MAbs, 4385G7 (IgG2b) and 4387A5 (IgG2a), had the strongest reactivity with the homologous A1 LOS, moderate reactivity with M978 (L8) LOS, but no reactivity with other LOS. The other six MAbs (4 IgM and 2 IgG3) reacted with A1 LOS but also with several or many of the 12 LOS. ELIZA inhibition at 50% showed that inhibitory activity of the LOS from strains A1 and BB431 (a group B strain) to the specific MAb 4387A5 was about 01-20 times greater than that of M978 (L8) LOS. When compared with Mab 2-1-L8 (L8) by Western blot analysis and ELIZA inhibition, the two specific MAbs recognized a different epitope in 3.6- kilodalton LOS of strains A1 and BB431. We propose this new epitope as L8a since the MAbs also reacted with M978 (L8) LOS. The expression of the L8a epitope in the A1 LOS requires a few monosaccharide residues in its oligosaccharide moiety and the fatty acid residues in its lipid A moiety also play a role. In whole cell ELIZA using the LOS prototype strains, the two specific MAbs bound only to L8 strains A1 and M978. By comparison, Mab 2-1-L8 not only bound to L8 strains but also had a low degree of reactivity with a few other prototype strains. These results suggest that the two specific MAbs can be used for LOS typing of N. meningitidis.